1. Field of the Invention
The present invention relates to a pressure sensitive circuit breaker (hereinafter referred to as "PSCB") to be used for ensuring the safety of a storage battery such as a secondary battery capable of being regenerated by re-charging. Particularly, the invention is concerned with a PSCB having a rectangular lid.
2. Description of the Prior Art
If a storage battery of this type is continued to be charged even after completion of charging or if it is charged with an electric current larger than a predetermined current, a trouble may arise in the storage batter, with evolution of gas therein, increase of pressure and temperature in the battery, consequent swelling of the battery, further, oozing of electrolyte caused by cracking and consequent bad influence on the device in which the battery is installed. Even if the result is not to such an extent, if the battery is continued to be used in such a state, the aforesaid swelling of the battery will proceed and may cause cracking or burst. Thus, as to a storage battery which has undergone some trouble, it is necessary to stop the use thereof immediately.
Among the devices for which the reduction of size has been required recently, there are many devices which are characterized by their small wall thickness. With this tendency, square or rectangular batteries have come to be used. Also in this case, reduction of thickness remains to be solved. On the other hand, in reply to a keen demand for safety, it is necessary to incorporate a safety valve and a current cut-off mechanism in such devices.
In a conventional PSCB referred to above, when the internal pressure of a storage battery increases due to some abnormal chemical change of a power generating element, the increased pressure acts on a safety valve, so that the safety valve is lifted and an electric conduction passage which connects a central projection with a lead through a lead stripper plate is stripped or broken. As the pressure further increases and reaches a predetermined level, a thin-walled portion of the safety valve is broken and a gas present in a battery case and having the increased pressure is released to the exterior through a hole formed in a lid.
However, the prior art described above involves a problem of complicated manufacture because a member for diaphragm is connected to the lid (a negative case) by caulking or welding. Particularly, in the case of welding, since spot welding is performed continuously, it has so far been necessary to ensure reliability of the welded portion so as to afford a satisfactory airtightness of the same portion and also necessary to inspect the welded portion.
Since two spaces (edges) for welding or caulking are needed in the transverse direction, the width of the diaphragm is narrowed relative to the width of the battery, resulting in that it becomes difficult to take a certain displacement of the diaphragm.
Even if the shape of the diaphragm is made elliptic or the like, it is not so effective because the diaphragm displacement is substantially restricted at a portion where the distance between the center and the peripheral edge is the shortest.